1. Field of the Invention
The present invention relates to a laminated ceramic capacitor having inner electrodes made of base metal Ni, which serves as a surface-mount component in an electronic device.
2. Description of the Prior Art
It is known that a laminated ceramic capacitor comprises electrodes sandwiched with dielectric material and can integrally be formed by ceramic manufacturing techniques which contribute to the minimal size and the large capacitance. Such a laminated ceramic capacitor is low in impedance and suitable for high frequency applications and also, will last semipermanently ensuring the operational reliability.
Particularly, a chip type laminated ceramic capacitor comprises a plurality of inner electrode layers for developing a capacitance, dielectric layers sandwiched between the inner electrode layers, and a pair of outer electrodes coupled to their associated inner electrodes for output of the capacitance The chip capacitor has no lead line and can directly be mounted to an assembly, thus encouraging a corresponding electronic device to be minimized in size and increased in function density and ensuring further development for advanced applications.
The most of known laminated ceramic capacitors employ paradium (Pd) of noble metal as a material of inner electrode while accepting the same dielectric material as of other conventional not-laminated plate or disk ceramic capacitors furnished with lead lines and specifically, are fabricated by firing in the air. The difference of the laminated type from the plate or disk type is to involve a green sheet of dielectric other than a powder-pressed sheet. For example, a dielectric material for ambient air firing is disclosed in Japanese Patent Laid-open Publication 49-34905 (1974), which is produced by adding 0.2 to 1.0 % by molecular weight of Gd.sub.2 O.sub.3, Sm.sub.O.sub.3, or Dy.sub.2 O.sub.3 to a powder of BaTiO.sub.3 for the purpose of fabrication of a high-frequency high-voltage ceramic capacitor having less change in the temperature of capacitance within a range from 0.degree. to 65.degree. C. Also, disclosed in the Publication 55-53007 (1980) is a dielectric material for ambient air firing which is adapted for attenuating the temperature hysteresis in a dielectric constant and formed by adding more than 0.1 % by atomic weight of at least one of La.sub.2 O.sub.3, Sm.sub.2 O.sub.3, Gd.sub.2 O.sub.3, and Dy.sub.2 O.sub.3 to a solid solution of barium titanate in which portions of ion component are replaced with Sr, Zr, or Sn, without changing the crystal structure of the solid solution. Those dielectric materials are also processed by firing in the air before being used. The requirements for improving conventional laminated ceramic capacitors will now be explained in detail. Simply, three key requirements are to minimize the size, to increase the capacitance, and to contribute to the low cost. The capacitance of a laminated ceramic capacitor is expressed by: ##EQU1## .epsilon.o: dielectric constant in vacuum .epsilon.s: relative dielectric constant of dielectric material
S: effective electrode area per layer PA1 d: thickness of dielectric layer PA1 n: number of layers
As apparent from the formula (1), for increasing the capacitance, it is most practical to enhance the dielectric capacity, increase the number of layers for ensuring a large surface area of the electrode, and reduce the thickness of each dielectric layer. All the efforts for achievement have been made by numbers of firms.
For minimizing the size, a chip is reduced from 3.2 mm.times.1.6 mm to 2.0 mm.times.1.25 mm and further from 1.6 mm.times.0.8 mm to 1.0 mm to 0.5 mm.
Reduction in the cost will then be described. The prior art laminated ceramic capacitor has inner electrodes which are made from a noble metal Pd and thus, will cost high as compared with other components: it is said, more than 70% the total production cost. The greater the capacitance, the higher the cost soars. Although excellent in the dielectric characteristics and the operational reliability, the laminated ceramic capacitor is yet disadvantageous in the cost of production, thus offering less marketability.
It is hence understood that the low cost or the reduction in the cost of fabricating inner electrodes is the most important factor among the foregoing three requirements. Both the reduction in size and the increase in capacitance depend on their means of production procedures rather than the properties of materials and have to be tackled for achievement simultaneously while corresponding measures are taken to reduce the cost.
There is a continuation of the trend towards employing base metals for inner electrodes in order to reduce the cost. More particularly, the inner electrode is made of nickel (Ni). However, its disadvantage to be overcome is notable as a base metal
Primarily, Ni is easily oxidized during firing in the air, failing to work as inner electrodes. Thus, it should be burned in the neutral or reducing atmosphere, preventing oxidation on the electrodes In such atmosphere, the foregoing dielectric material is reduced lowering the insulation resistance, because barium titanate (BaTiO.sub.3), a primary component of the dielectric material becomes semiconductive. It is believed that oxygen defect in the atmosphere partially shifts Ti.sup.4+ to Ti.sup.3+ causing electronic conduction in hopping (refer to "Innst Electr Engs. Paper 3634" by Glaister R. M., in 1961).
For use with the inner electrodes of Ni, it is thus essential to employ a dielectric material which is not reduced under low-oxygen atmosphere.
Such a novel dielectric material as remaining unreduced in the neutral or reducing atmosphere is described in U.S. Pat. No. 3,920,781, and "High permittivity ceramics sintered in hydrogen" to J. M. Herbert, issued in 1963, in which transition metal oxide or MnO.sub.2 is preferably provided for addition. The disadvantages are that the dielectric constant is low and that variations in the Curie point are notably great depending on the temperature, time, and atmosphere during firing. Also, another dielectric material is disclosed in U.S. Pat. No. 4,115,493, which is a barium titanate composition having a structural formula of: EQU {(Ba.sub.1-x Ca.sub.x)O}.sub.m .multidot.(Ti.sub.1-y Zr.sub.y)O.sub.2
where m, x, and y are expressed as: EQU 1.005.ltoreq.m.ltoreq.1.03 EQU 0.02.ltoreq.x.ltoreq.0.22 EQU 0&lt;y.ltoreq.0.20
A further dielectric material is disclosed in Japanese Patent Laid-open Publication 58-143515 (1983), in which a laminated ceramic capacitor employing base metal electrodes is provided with dielectric having a formula of: EQU (Ba.sub.1-x Y.sub.x)A(Ti.sub.1-y Zr.sub.y)B+z wt % M
where M is an oxide of Mg, Cr, V, Mn, Sn, In, or W and x, y, A/B, and z are expressed as: EQU 0.005.ltoreq.x.ltoreq.1.03 EQU 0.ltoreq.y.ltoreq.0.3 EQU 1.00&lt;A/B.ltoreq.1.05 EQU 0&lt;z.ltoreq.1.0
The resultant dielectric constant is however not high enough, contributing to the large capacitance unsuccessfully.
Secondly, a method of production procedures is also an important factor for fabricating a laminated ceramic capacitor having inner electrodes of Ni base metal; particularly, a process of firing. Some organic materials are commonly used in production of laminated ceramic capacitor in the form of organic binder, plasticizer, and solvent all added during making a green sheet and also, organic binder solvent contained in a paste for inner electrode. While the solvent is dispersed off by drying, both the organic binder and plasticizer remain unaffected prior to firing. During a common firing process in the air, such organic components are eliminated by burning out at a firing step. However, in the neutral or reducing atmosphere and more specifically, the atmosphere in which Ni of the electrode remains unoxidized, the elimination of organic components is found difficult. A solution to this problem is disclosed in U.S. Pat. No. 4,714,570, in which a method of metallizing base metal electrodes is introduced employing base metal oxide such as NiO, Fe.sub.2 O.sub.3, or CoO as a starting material of the electrodes. It features the successive steps of removal of binder in the air, reduction in hydrogen gas, and firing in nitrogen gas. Non of metallic elements such as Ni, Co, or Fe is used because of preventing a change in volume which results from oxidation during binder removal and will cause cracking in the dielectric layer. This method covers both the procedures of metallizing base metal and eliminating organic components.